1. FIELD OF THE INVENTION
The present invention relates to a vacuum circuit breaker which employs a vacuum interrupter. The vacuum circuit breaker cuts off a current from flowing through a circuit in excess of a prescribed value, so as to protect the circuit.
2. DESCRIPTION OF THE RELATED ART
A vacuum circuit breaker recovers the electrical insulation between its main electrodes at the zero point of current and cut s off the current, thereby protecting a circuit from any overcurrent. By way of example, FIG. 8 of the accompanying drawings illustrates the circuit arrangement of a DC (direct-current) vacuum circuit breaker (also termed "DC circuit breaker") having hitherto been conventional, while FIG. 9 illustrates the operating principles of the DC circuit breaker.
Referring to FIG. 8, the DC circuit breaker 1 is constructed of a vacuum interrupter 2, a commutating capacitor 5, a commutating reactor 6, a trigger gap 8, an electromagnetic repulsion coil 3, a short-circuit ring 4, an overcurrent tripping device 7, and a zinc-oxide (ZnO) non-linear resistance element 9.
In the prior-art circuit breaker 1 constructed as stated above, the commutating capacitor 5 is previously charged by a charging device with such a polarity the that stored charge becomes negative on the side of a DC power source 10 and positive on the side of a load 11 as shown in FIG. 8. When an overcurrent I.sub.0 has flowed through the main circuit of the circuit breaker 1, it is detected by the overcurrent tripping device 7. Simultaneously with the detection, the overcurrent tripping device 7 generates a signal by which the electromagnetic repulsion coil 3 is excited to induce an electromagnetic repulsive force between it and the short-circuit ring 4. At a time t.sub.0 indicated in FIG. 9, the movable electrode 2b of the vacuum interrupter 2 parts or separates from the fixed electrode 2a thereof, and an electric arc strikes across the movable electrode 2b and the fixed electrode 2a. On this occasion, the electric arc undergoes an axial magnetic flux 9 (shown in FIG. 9) generated axially of the vacuum interrupter 2 by the fixed electrode 2a (constituting first magnetic flux generation means, and being a contact) and the movable electrode 2b (constituting the first magnetic flux generation means, and being another contact) themselves. The electric arc is therefore kept stable across both the electrodes 2a and 2b.
At a time t.sub.2 after the opening of the vacuum interrupter 2, the trigger gap 8 is ignited or sparked by a signal which is delivered from the overcurrent tripping device 7. Then, a closed circuit extending along the commutating capacitor 5--commutating reactor 6--trigger gap 8--vacuum interrupter 2 is established. Thus, the charge stored in the commutating capacitor 5 beforehand is discharged, and a reverse current I.sub.C1 flows in a direction reverse to that of the current of the main circuit of the circuit breaker 1.
Owing to the reverse current I.sub.C1, a current (I.sub.0 +I.sub.C1) flowing through the vacuum interrupter 2 reaches the zero point of the currents at a time t.sub.3. Then, the electric arc in the vacuum interrupter 2 is extinguished, and the main circuit current is commutated to a circuit path consisting of the commutating capacitor 5--commutating reactor 6--trigger gap 8.
Consequently, energy having been stored in the inductance of the load (11) side changes into energy for charging the commutating capacitor 5, so that the terminal voltage of the commutating capacitor 5 rises. When terminal voltage has reached the operating voltage of the ZnO non-linear resistance element 9, this non-linear resistor 9 conducts to discharge the stored charge of the commutating capacitor 5. Then, the breaking operation the circuit breaker 1 is completed.
With the prior-art technique, as illustrated in FIG. 9, the attenuation rate of the axial magnetic flux .phi..sub.0 generated between the electrodes 2a and 2b by these electrodes themselves on the basis of the overcurrent I.sub.0 flowing through the main circuit is low with respect to the period of the reverse current I.sub.C1 which begins to be introduced at the time t.sub.2. As indicated at symbol .phi..sub.0 ', therefore, a flux .phi..sub.r remains even at that zero point of the sum current (I.sub.0 +I.sub.C1) of the main circuit which is developed at the time t.sub.3 by the reverse current I.sub.C1.
On account of the residual flux .phi..sub.r, charged particles existing between the electrodes 2a and 2b are hindered from diffusing radially of the vacuum interrupter 2 at the current zero point at the time t.sub.3, and a recovery rate the insulation between these electrodes lowers. As a result, the electrodes 2a and 2b fail to withstand a transient recovery voltage, and they strike an electric arc again. Thus, the breaking performance of the circuit breaker 1 is suppressed disadvantageously.